The majority of the boats were fitted out by Frederick Mitchell & Sons of Poole. (see the dedicated page for Mitchells) but it is probable that some hulls were fitted out at Southern Boat Building Co Ltd , Southampton ( see Rival Owners site for history of this builder. One of their yachts, known as the “Rival” class was unfortunately made famous recently as the type of yacht that the Chandlers were sailing when the Somali pirates tragically kidnapped them. Thankfully they are now released).
For the proof of the pudding regarding Mitchells and Southern, have a look at the two pictures below which were given to me by David Coxon, to whom I owe a debt of gratitude for supplying a lot of information/pictures on this site.